title: Accurate calibration of mercury vapour measurements.
abstract: Almost all measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Setting aside the uncertainty in the vapour pressure of mercury at a given temperature, the accuracy of vapour phase mercury determinations depends critically on fully understanding the operation and sensitivities of the mercury bell-jar apparatus. This paper discusses the thermodynamic and kinetic considerations that must be taken into account when using the bell-jar apparatus, provides the theoretical basis for understanding the operation of the bell-jar, and presents experimental data demonstrating the systematic biases which may be obtained if the bell-jar is used incorrectly. These biases depend on the temperature difference between the mercury vapour in the bell-jar and the syringe used to remove the mercury vapour from the bell-jar, but they may be well in excess of 10% under some operating conditions. The results from this study have been used to propose best practice solutions for mercury vapour calibrations using the bell-jar.
authors.affiliation: Analytical Science Team, National Physical Laboratory, Teddington, Middlesex, UK. richard.brown@npl.co.uk
authors.forename: Richard J C
authors.initials: RJ
authors.lastname: Brown
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Brown
mesh_terms: D000395:Air Pollutants, Occupational / Q000032:analysis*; D016902:Air Pollution, Indoor / Q000032:analysis*; D002138:Calibration; D004784:Environmental Monitoring / Q000295:instrumentation / Q000379:methods; D008628:Mercury / Q000032:analysis*; D013816:Thermodynamics; D014835:Volatilization
chemical_list: D000395:Air Pollutants, Occupational; D008628:Mercury